Andreas Maurer wrote:
On Thu, 03 Jun 2010 19:43:46 -0500, brian whatcott
wrote:


Specifically what I am saying is that unless you are exceeding the
Maximum Winch Launch speed of the glider it should be impossible to
overstress the wings as the wing will stall before exceeding maximum
structural load of the aircraft.

Brian

This is a perfectly sound argument in many phases of flight.
But a tow capable of exerting enough force on a stalled structure
will certainly break it. A winch need not relax when a wing stalls....
Visualize an airframe being hauled along at an AoA of 90 degrees,
to exaggerate this point...

This won't break the wing spars either.

As long as the IAS is below VA (in other words: in the green range of
the airspeed indicator), no control input at all can damage the glider
- the definition of VA is that this is the speed under which a high
speed stall will prevent exceeding the maximum g-load. So, as long as
you are slower than VA, you are safe.

(Of course - combined control inputs can indeed break the structure
even below VA, usually by torsional load on the tail boom by a
deflected rudder at speeds close to VA combined with significant yaw
angle.)


The problem of exceeding the Maximum Winch Launch speed is not the
wing, but the attachment of the tow hook to the fuselage structure.
Pretty hard to explain why you landed with the belly hook missing...


Cheers
Andreas
Bye
Andreas

Better to think twice and write once.

For instance, how does your ASI indicate at 90 deg AoA ?
If an airframe is designed for 1.5 (the design factor)
times limit load (often 4 g)
then if it is pulled with a force exceeding this, at high AoA,
wing breakage is guaranteed.

Sincerely

Brian W